I don't think you'd need a custom ROM. You could just factory reset it and only install apps that's you'd use for music. I can't remember if you can remove bloatware unrooted, but you could set up a clean home screen and ignore all the unnecessary apps.

Alternatively, I picked up an old FiiO X3 from 2016 for like $100. The battery is way better and the audio quality is fantastic. It's nice having a dedicated device that I can navigate in my pocket, and I dig the iPod Classic vibe.

You should be able to use adbshell with shizuku (both in f-droid) to remove whatever you won't use. I doubt there's any custom Roms that'll help here though. If you're willing to root the phone then you can probably find some way to get what you want.

If you have any music on your computer, an old clickwheel iPod is the way to go

Well what phone is it? There are tons of music playing programs on f-droid. Why not just run one? I've been using Vanilla Music. I don't like it that much, but don't feel like derping around trying more and more of them. Vanilla is better than some others that I tried.

Doesn't it just work as-is? Upload your songs or use Spotify pro to download what you want locally and then just dance away.

custom rom is really difficult for a lot of phones.

I have the same idea on my mind for a while but seeing how messy the Android space is (mix of gamers and tinkerers), how heavy it is to compile a ROM (250Gb of data), knowing how shity vendor shares - if any - drivers... I feel that it would be better to throw away (in a recycle park of course) that ie: samsung s4 an buy a basic music player.

it's sad that it is so difficult to build a minimal ROM for an obsolete but still powerful smartphone.